1. Field of the Invention
The present invention is related to wireless networks, where nodes may communicate via different communication standards and technologies such as IEEE 802.11 or WiFi Direct.
2. Discussion of the Background
The medium access control (MAC) standards that are used in wireless networks that operate in a distributed fashion (e.g., mesh, ad hoc) offer a sub-optimal temporal and spatial utilization of the wireless medium. For example, the fundamental distributed medium access method of the IEEE 802.11 Wireless Local Area Network (WLAN) MAC standard, known as the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), suffers from the so-called “exposed node” problem, which happens when a node defers its transmission due to a neighboring transmitter even though its transmission would not interfere with the existing one due to the positions of the corresponding two receivers.
A wireless node can successfully decode any received signal if its reception strength is greater than the noise (and interference, if exists) level by a certain ratio. To ensure that this ratio is met for a receiver, the IEEE 802.11 Distributed Coordination Function (DCF) MAC standard allows only a single data communication at a time in the wireless medium shared by a group of nodes. Every other node that hears an ongoing transmission in the medium defers its own transmission. In other words, every potential transmitter that is a neighbor of an active (i.e., currently transmitting) transmitter must always defer its own transmission.
In FIG. 1, the IEEE 802.11 DCF MAC standard compliant transmitter T2 always defers its transmission to node R2 when there is an ongoing communication between nodes T1 and R1. In this topology, node R2 is outside of the communication range of T1. (The dot-shaded circular region centered at T1 is drawn to represent the communication range of T1. In practice, the communication range of a node forms an imperfect shape instead of a circle, with an asymmetric reach in every direction due to the wireless propagation characteristics in a given environment.) Similarly, R1 is outside the communication range T2. In this topology, in fact, the transmissions T1-to-R1 and T2-to-R2 can be performed concurrently as neither of their corresponding receivers can hear from the other transmitter but only from their target transmitter (the one they are trying to receive from).
This impact can be observed in FIG. 2 as well. In this figure, a dotted line between nodes indicates that the nodes are within communication distance of each other, and a straight (directional) line indicates an ongoing communication. While AP 202 is transmitting unicast packets to AP 203, AP 208 decodes the MAC headers of 202's data packets and thus learns about the active communication in the medium: its source, target, and duration. According to the IEEE 802.11 MAC standard, AP 208 cannot transmit until the ongoing communication is complete. However, in this topology, a transmission from 202 to 203 would not interfere with the transmission from 208 to 207, because node 207 cannot receive signals from transmitter 202, and node 203 cannot receive signals from 208. Cocktail Party is designed in order to alleviate the capacity under-usage introduced by the unneeded withdrawal of transmission.